An adequate theory of language acquisition must take account of both the internal and environmantal constraints on the acquisition process. Recent work on the syntactic environment alone indicates that it does not function as a model from which to induce language directly. But, the communicative environment consists of other sources of information that can act as constraints on the child's possible hypotheses about language. The child's cognitive capacities and strategies for dealing with interactions further constrain interpretations of language. Previous research has described aspects of the communicative environment and has given evidence for early action-based strategies. The proposed research includes (1) studies examining both the normal and the language-impaired child's ability to process and integrate various kinds of contextual information relevant to understanding messages; (2) enrichment studies using selected linguistic and non-linguistic aspects of interactions to assess their role in the development of both syntactic and conversational skills; (3) studies with children, 1 to 3 years of age, investigating the importance of gesture for understanding and the development of what as a stop-action marker in varied contexts. The studies taken together are attempts to provide evidence for mechanisms of language acquisition that depend on both internal and external constraints.